Thermal overload device containing a polymer composition containing thermally exfoliated graphite oxide and method of making the same

ABSTRACT

A thermal overload device containing a polymer composite, which contains at least one polymer and a modified graphite oxide material, containing thermally exfoliated graphite oxide having a surface area of from about 300 m2/g to 2600 m2/g, and a method of making the same.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.14/515,019, filed Oct. 15, 2014, now allowed, which was a Continuationof U.S. patent application Ser. No. 13/077,070, filed Mar. 31, 2011, nowU.S. Pat. No. 8,891,247, which was a Divisional of U.S. patentapplication Ser. No. 12/194,030, filed Aug. 19, 2008, now U.S. Pat. No.8,048,214, which was a Continuation of U.S. patent application Ser. No.11/249,404, filed Oct. 14, 2005, now U.S. Pat. No. 7,658,901, the entirecontents of each of which are hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH

This invention was made with government support under Grant No.CMS0609049 awarded by the National Science Foundation and under GrantNo. NCC-1-02037 awarded by NASA Langley Research Center. The governmenthas certain rights in the invention.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a high surface area material based onmodified graphite oxide.

Discussion of the Background

There has been considerable interest in the area of nanoparticle-filledpolymer composites (NCs), in particular composites in which thenanoparticle has dimensions comparable to those of the polymer chains,has a high aspect ratio of more than 100 and is uniformly dispersed inthe polymer matrix. There are several filler materials that have beenextensively studied for improvement of mechanical properties, electricaland thermal conductivity of polymer composites, for example, fractalagglomerated nanoparticles (silica and carbon black), carbon nanotubes(CNTs), inorganic clays and alumina silicate nanoplates.

Initial attempts at producing nanoparticle-filled polymer compositesoften resulted in materials with inadequate nanoparticle dispersion anddegraded mechanical properties. Although often impractical forindustrial applications, small-scale dispersion methods involvingsolvent- or monomer based processing have occasionally yielded NCs withmultifunctional capabilities and improved mechanical properties. Severalproblems remain that affect the development of NCs with consistentproperties that are viable for use in real world applications: (1) manyof the nanoparticles used are expensive (e.g., CNTs); (2) often chemicalor mechanical manipulations must be performed to achieve good dispersionthat are impractical for large-scale commercial production; and (3)problems of the interfacial energy mismatch of inorganic nanofillerswith hydrocarbon polymer matrix phases result in processing andmechanical property difficulties.

A significant amount of work has been done with nanoclays.Nanoclay-reinforced composites have shown enhancements in stiffness,glass transition temperature, barrier resistance, and resistance toflammability in a variety of polymer systems. Nanoclays are also highaspect ratio nanoplates that are, like graphene, derived frominexpensive commodity materials (clays) and thus appropriate forcomparison with the projected graphene polymer composites of the presentinvention. The in-plane modulus of clays should be similar to that ofmica, which is ˜178 GPa, significantly lower than the 1060 GPa value ofgraphene (value from graphite in-plane). Recent studies point out thatthe hydrophilicity of clays makes them incompatible with most polymers,which are hydrophobic. One approach is to render the clays organophilicthrough a variety of approaches (amino acids, organic ammonium salts,tetra organic phosphonium). Such clays are called “organoclays.” Thesematerials have suffered from the cost of the added interfacial modifiersand the instability of these modifiers under processing conditions. Inaddition, it has been difficult to homogeneously disperse theseorganoclays in polymer matrices.

Carbon nanotubes have also generated significant interest asnanofillers. They have good mechanical properties and large aspectratios, and their surfaces should be more compatible with hydrocarbonpolymers than clay-based nanofillers. As a nanofiller, CNTs have severallimitations, one of which is their cost of production. Since they aremade in a gas-phase process, the production costs are more expensivethan solution-based processes operating at high density. The productionof single wall carbon nanotubes (SWCNTs) requires the addition of metalcatalysts that must be removed to produce pure SWCNT materials, orresults in the presence of heavy metal contaminants in the finalmaterials if not removed.

Graphite is a “semi-metal,” and recent efforts have demonstrated thatextremely thin (few layers thick) nanoplates obtained from highlyoriented pyrolytic graphite (HOPG) are stable, semimetallic, and haveexceptional properties for metallic transistor applications.

Even though graphene sheets have the same sp² carbon honey combstructure as carbon nanotubes (CNTs), until now, it has not beenpossible to effectively produce the highly dispersed, thin sheets neededto make graphene applications possible.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide exfoliatedgraphite oxide.

It is another object of the present invention to provide a method formaking exfoliated graphite oxide sheet, in particular thermallyexfoliated graphite separated down to individual graphene sheets.

It is another object of the present invention to provide a materialbased on modified graphite that is appropriate, for example, as ananofiller for polymer composites, a conductive filler for composites,an electrode material for batteries and ultracapacitors, as a filler toimprove diffusion barrier properties of polymers, and as a hydrogenstorage material.

It is another object of the present invention to provide a fillermaterial that has dimensions comparable to those of polymer chains, hasa high aspect ratio of more than 100 and can be uniformly dispersed in apolymer matrix.

It is another object of the present invention to provide a materialbased on modified graphite that is electrically conductive and canconfer electrical conductivity when formulated with a polymer matrix.

It is yet another object of the present invention to provide a materialbased on modified graphite that has a high aspect ratio so that it canperform as a barrier to diffusion when incorporated in a polymercomposite.

This and other objects have been achieved by the present invention thefirst embodiment of which includes a modified graphite oxide material,comprising a thermally exfoliated graphite oxide (TEGO) with a surfacearea of from about 300 m²/g to 2600 m²/g, wherein said thermallyexfoliated graphite oxide displays no signature of graphite and/orgraphite oxide, as determined by X-ray diffraction (XRD).

In another embodiment, the present invention relates to a method ofmaking the above TEGO.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates XRD patterns of graphite and graphite oxide preparedby oxidation for different durations.

FIG. 2 shows selected area electron diffraction (SAED) patterns of GOoxidized for 96 hours, and the structure in the diffraction rings fromstack spacing in GO.

FIG. 3 illustrates a solid-state 13C-NMR spectra of GO, with the sampleoxidized for 96 hours.

FIGS. 4a and 4b illustrate XRD patterns of TEGO and GO samples preparedby oxidation for 96 and 24 hours and rapidly expanded at 1050° C. Theincompletely oxidized GO in FIG. 4b produces a more pronounced peak at2Θ≈26.50 after heat treatment.

FIG. 5 shows a Selected Area Electron Diffraction (SAED) pattern of TEGOproduced from fully oxidized GO (96 hours) with no structure in thediffraction rings. The structure of TEGO is found to be totallydisordered commensurate with the XRD information in FIGS. 4a and 4b

FIG. 6 shows BET surface area of TEGO samples prepared by heating GOsamples at different temperatures for 30 seconds.

FIGS. 7A and 7B show (7A) a XRD pattern of EG and (7B) a SEM image ofEG, respectively.

FIG. 8 shows an atomic force microscope (AFM) image illustrating thethin, wrinkled platelet structure. Superimposed on the image is theheight profile taken along the indicated line scan. The average heightis ˜2.8 nm.

FIG. 9 shows the high-resolution X-ray photo electron (XPS) spectra ofTEGO.

FIG. 10 shows digital image of TEGO/PMMA samples at differing weightfraction loadings.

FIGS. 11A and 11B show (11A) Thermal gravimetric analysis (TGA) tracesshowing the thermal degradation properties of differentnanofillers-reinforced PMMA composites and (11B) Storage modulus vs.temperature of different nano fillers in PMMA, respectively.

FIGS. 12A and 12B show Scanning electron microscope (SEM) images ofTEGO-PMMA fracture surface. By using a high acceleration voltage (20kV), the sub-surface morphology of TEGO nanoplates can be observed. Thepersistent wrinkled nature of the TEGO nanoplates within the compositeprovides for better interaction with the host polymer matrix.

FIG. 13 shows normalized tan delta peaks from the dynamic mechanicalanalysis (DMA) showing a ˜35° C. increase in T_(g) (even at the lowest0.05 wt % loading) for TEGO/PMMA over those observed for SWCNT/PMMA orEG/PMMA nanocomposites.

FIG. 14 shows a schematic of the current distribution through thecomposite sample resulting from a voltage bias applied between two metalelectrodes (light grey).

FIG. 15 shows electrical conductivity of TEGO/PMMA nanocomposites as afunction of filler content based on transverse AC measurements.

FIGS. 16A, 16B, and 16C show in (16A) a summary of thermomechanicalproperty improvements for 1 wt % TEGO-PMMA compared to SWCNT-PMMA andEG-PMMA composites. All property values are normalized to the values forneat PMMA and thus relative to unity on the scale above. Neat PMMAvalues are E (Young's modulus)=2.1 GPa, T_(g) (glass transitiontemperature)=105° C., ultimate strength=70 MPa, thermal degradationtemperature=285° C. (16B and 16C) SEM images of EG-PMMA vs. TEGO-PMMA,respectively: The size scale (nanoplate thickness) and morphology(wrinkled texture) of the TEGO nanoplates as well as their surfacechemistry lead to strong interfacial interaction with the host polymeras illustrated by the fracture surface (16C). In contrast, simpleexpanded graphite exhibits thicker plates with poor bonding to thepolymer matrix (16B).

FIG. 17 shows storage modulus vs. temperature response of differentweight % of TEGO in TEGO/PMMA composite.

FIG. 18 shows a RT storage modulus (GPa) vs. weight % of TEGO/PMMAcomposite.

FIGS. 19A and 19B show a SEM picture of (19A) 1 weight % and (19B) 5weight % of TEGO/PMMA composites, respectively.

FIG. 20 shows normalized tan delta with temperature sweep of differentweight % of TEGO/PMMA composite.

FIG. 21 shows thermal degradation of TEGO/PMMA composite by TGAanalysis.

FIG. 22 shows real Z vs. frequency response of TEGO/PMMA composite.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The relatively low cost of graphite as compared to CNTs make exfoliatedgraphite an attractive material. The use of graphite nanoplatelets(GNPs) is advantageous because of the chemistry of the graphene andgraphene-like sheets compared to clay nanoplates. The inventors of thepresent invention have found that exceptionally rich chemistry of carboncan be utilized for interface engineering in composites and also formany other possible application areas, such as the use of grapheneplates in nanoelectronics and sensors. Graphene and graphene-like platesare hydrophobic and thus compatible with a broad range of polymers andother organic materials, including proteins, and DNA. Additionally, itis possible to “tune” the wettability of graphene sheets throughchemical coupling with functional groups.

Graphite or graphene sheets interact with each other through van derWaals forces to form layered or stacked structures. Theoretically,graphene sheets may have a surface area as high as 2,600 m²/g, sincethey are composed of atomically thick layers. Graphite has anisotropicmechanical properties and structure. Unlike the strong sp² covalentbonds within each layer, the graphene layers are held together byrelatively weak van der Waals forces. Due to this anisotropy, graphitehas different properties in the in-plane and c-axis direction.

The chemical modification of graphite to intercalate and oxidize thegraphene sheets has been described in the literature. Intercalation, aprocess in which guest materials are inserted into the “gallery” of hostlayered materials, creates a separation of these sheets beyond the 0.34nm spacing of native graphite. Layered materials that form intercalationcompounds include graphite, boron nitride, alkali metal oxides andsilicate clays. Among these materials, graphite and boron nitride arethe only solid layered materials that are composed of atomically thinsheets of atoms and are unique in their ability to form “stages” inwhich a monolayer of guest intercalant is separated by n multilayers ofhost to form “stage-n” compounds. The intercalation process usuallyinvolves chemical reaction and charge transfer between the layered hostmaterial and reagent, resulting in the insertion of new atomic ormolecular intercalant layers. Due to its amphoteric nature, graphite canreact with reducing or oxidizing agents, leading to the formation ofeither donor or acceptor type graphite intercalation compounds (GICs).For donor GICs, the intercalates (anions) donate electrons to the hostlayers, whereas for acceptor GICs the intercalates (cations) extractelectrons from the host layers. The process of the present inventionbegins with, and is dependant on, the substantially completeintercalation of graphite to form stage n=1 graphite oxide.

The effect of intercalation on the bond lengths of the carbon atoms inbounding layers also depends on whether donors or acceptors areconsidered. Furthermore, with alkalis there is a small expansion overthe pristine value of 1.420 Å that is roughly proportional to thevalence and inversely proportional to the stage index and ionic radiusof the metal. The intercalation process may result in deformation orrumpling of the carbon layer by the intercalant. A local buckling of thecarbon layers may also occur.

The result of partial oxidation of graphite produces graphite oxide(GO). Many models have been proposed to describe the structure ofgraphite oxide. However, the precise structure of GO is still an area ofactive research.

A process of making expanded graphite materials with an accordion or“worm-like” structure has been proposed. These materials have manyapplications, including electromagnetic interference shielding, oilspill remediation, and sorption of biomedical liquids. The majority ofthese partially exfoliated graphite materials are made by intercalationof graphite with sulfuric acid in the presence of fuming nitric acid toyield expanded graphitic material. These expanded materials are thenheated to yield an increase in the c-axis direction. While thesematerials are sometimes referred to as “expanded graphite” or“exfoliated graphite,” they are distinct from the TEGO of the presentinvention. For these “worm-like” expanded graphite oxide materials, theindividual graphite or GO sheets have been only partially separated toform the “accordion” structures. Although the heating results in anexpansion in the c-axis dimension, the typical surface area of suchmaterials is in the order of 10-60 m²/g. Both the surface area below 200m²/g and the presence of the 0002 peak of the pristine graphitecorresponding to a d-spacing of 0.34 nm are indicative of the lack ofcomplete separation or exfoliation of the graphene sheets. While theterm “graphene” is used to denote the individual layers of a graphitestack, and graphite oxide denotes a highly oxidized form of graphitewherein the individual graphene sheets have been oxidized, graphene willbe used to denote the layered sheet structure that may be in a partiallyoxidized state between that of native graphene and graphite oxide.

The present invention relates to a material based on modified graphitethat is appropriate, for example, as a nanofiller for polymercomposites, a conductive filler for composites, an electrode materialfor batteries and ultracapacitors, as a filler to improve diffusionbarrier properties of polymers, and as a hydrogen storage material. Thegraphite nanoplatelet (GNP) material is distinct from previous graphiticmaterials, which lack one or more of the attributes required for asuccessful nanofiller. Also, the present invention relates to a materialbased on modified graphite that is electrically conductive and canconfer electrical conductivity when formulated with a polymer matrix.The present invention further relates to a material based on modifiedgraphite that has a high aspect ratio so that it can perform as abarrier to diffusion when incorporated in a polymer composite.

More specifically, the present invention relates to a novel materialbased on exfoliation of oxidized graphite by a novel process. Theinitial step of the process is the intercalation and oxidation ofnatural graphite to form oxidized graphite, or graphite oxide (GO). Theinitial step causes the spacing between graphene layers to expand withloss of the native 0.34 nm spacing. During the expansion process, a peakassociated with the 0.34 nm spacing as seen in XRD patterns willdisappear and simultaneously a peak associated with a 0.71 nm spacingwill appear. The best measure for substantially complete intercalationand oxidation of graphite is the disappearance of the 0.34 nmdiffraction peak and its replacement with only the 0.71 peak. So far theliterature has not reported such complete intercalation and oxidation ofgraphite. Substantially complete intercalation is represented, forexample, in FIGS. 4 and 5. The resulting functional groups on GO, suchas hydroxyl, epoxy, and carboxylic groups, alone or in combination,facilitate the retention of water molecules in the galleries between theGO layers. Rapidly heating the GO (after the 0.34 nm XRD peak iscompletely replaced by the 0.71 nm peak) results in superheating andvolatilization of the intercalants, imbibed solvent, such as water andmixture of water with water-soluble solvents, and evolution of gas, suchas CO₂, from chemical decomposition of oxygen-containing species in thegraphite oxide. These processes, individually and collectively, generatepressures that separate or exfoliate the GO sheets. In the context ofthe present invention, the term “exfoliate” indicates the process ofgoing from a layered or stacked structure to one that is substantiallyde-laminated, disordered, and no longer stacked. This procedure yieldsdisordered GO sheets which appear as a fluffy, extremely low densitymaterial with a high surface area. Disordered GO shows no peakcorresponding to 0.71 nm in the X-ray diffraction pattern. During rapidheating in an inert atmosphere, the GO is partially reduced and becomeselectrically conductive. The rate of heating can be at least about 2000°C./min, preferably higher than 2000° C./min. The inert atmosphere is notparticularly limited as long the gas or gas mixture is inert.Preferably, nitrogen, argon or mixtures thereof are used. In addition,reducing atmospheres may be used, such as carbon monoxide, methane ormixtures thereof. The TEGO can be readily dispersed in polar solventsand polymers, and can be used, for example, in composites asnanofillers, in ultracapacitors, as dispersants, and as hydrogen storagematerials.

The water enters through interactions with the polar oxygenfunctionality and the ionic intercalants. But water is not anintercalant.

The water retention in the galleries between the water molecules may be1 to 500%, preferably 1 to 300%, and most preferably 1 to 100% by weightbased on the total weight of the GO. The water retention includes allvalues and subvalues there between, especially including 5, 10, 20, 40,60, 80, 100, 150, 200, 250, 300, 350, 400, 450% by weight based on thetotal weight of the GO. The water used is preferably deionized water,preferably water having a resistivity between 100 and 0.2 MΩ/cm, morepreferably between 50 to 0.2 MΩ/cm, most preferably between 18 to 0.2MΩ/cm. The resistivity includes all values and subvalues there between,especially including 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,15, 16 and 17 MΩ/cm.

The solvent for conducting the oxidation of graphite to produce graphiteoxide is not particularly limited. While the preferred medium is water,co-solvents or additives can be used to enhance wetting of thehydrophobic graphite flakes. Solvents and/or additives may be used aloneor in combination. Preferred additives include alcohols such asmethanol, ethanol, butanol, propanol, glycols, water soluble esters andethers, surfactants such as non-ionic ethylene oxide, propylene oxideand copolymers thereof, alkyl surfactants such as the Tergitol familysurfactants, or the Triton family of surfactants, or surfactants withethylene oxide and propylene oxide or butylene oxide units. Examples ofthese include the Pluronic or Tetronic series of surfactants. Cosolventsand surfactants can be used at levels from 0.0001 to 10 wt. % of thesolution phase. The amount of cosolvents and surfactants includes allvalues and subvalues there between, especially including 0.0005, 0.001,0.005, 0.01, 0.05, 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6,6.5, 7, 7.5, 8, 8.5, 9 and 9.5% by weight based on the solution phase.

The polar functional groups on TEGO, are preferably hydroxyl, epoxygroups and carboxylic acid groups or their derivatives. These polargroups can be functionalized using molecules that are reactive towardthese polar functional groups. More than one type of functional groupsmay be included. For example, alkyl amines and dialkyl amines can beused to add hydrophobicity to the surface by reaction to epoxides, andcan be used to covalently crosslink the TEGO surfaces. Acid chloridescan react with hydroxyls to add alkyl groups. Reactions of amines orhydroxyls with carboxylic acids can be used to attach groups to make thesurface more hydrophobic by adding alkyl groups. The surfaces can bemade more hydrophilic by adding ethylene oxide, primary and secondaryamines and acid functionality using, for example the chemistries listedabove. An important class of modification includes the grafting ofspecies on the surface to increase the cohesive interactions between thefiller surface and polymer matrices. These grafting agents can includelow molecular weight analogs of the polymer matrix phase, or polymerswith the same composition as the matrix phase that have reactivefunctionality. These might include polyethylene or polypropylenecopolymers of vinyl acetate or maleic anhydride or their mixtures toinduce compatibility between TEGO and olefin polymers.

Intercalants include but are not limited to inorganic acids or theirsalts, alone or in mixtures, preferably HNO₃, H₂SO₄, HClO₄, KClO₄.

Gases evolved during heating include water vapor from bound waterbetween the GO layers, oxides of sulfur SO_(x) and H₂S from intercalatedsulfates not removed by washing, oxides of nitrogen NO_(x) if nitratesare used as intercalants, CO₂, CO, and C_(n)H_(m)O_(o) species frompartial reduction and elimination of oxygenated species from the GOprecursor. X, m, n, o are numbers, preferably integers. More than onekind of gas may evolve during the heating. In one embodiment, IR-spectraof the decomposition products in the vapor phase during exfoliation showthe presence of SO₂, CO₂ and water in the unwashed GO sample and onlyCO₂ and water in the washed sample. The SO2 arises from decomposition ofthe intercalated sulfate ions, and the CO₂ comes from decomposition ofoxygenated species on GO. Minor amounts of higher carbon number evolvedgaseous products may be produced. And if nitrate intercalants are usedthere may be NOx species released.

The rapid heating in an inert gas atmosphere occurs as follows. Rapidheating of the GO precursor is required to successfully produce TEGO. Ifthe temperature increase is too slow then evolved gases can escapethrough the lateral channels between GO sheets without buildingpressures great enough to exfoliate the GO. Inadequate heating rates canoccur because the temperature gradient between the sample and the ovenis too low, the temperature gradient is applied too slowly, or too largeof a sample is processed at one time so that heat transfer resistancesinside the GO bed result in slow heating of the interior of the samplebed. Temperature gradients on the order of 2000° C./min produce TEGOmaterials of surface areas as high as 1500 m²/g. This corresponds to 30second heating times in a 1050° C. tube furnace. Heating rates of 120°C./min produced TEGO samples with only 500 m²/g. Gradients even higherwill produce even greater exfoliation, with the limit being thetheoretical maximum value of 2600 m²/g. In order to attain the maximumsurface area, it may necessary to colloidally disperse TEGO in polarsolvent and measure the surface area by adsorption methods in solution.This will ensure that all the surface area is available as a result ofcolloidal dispersion. In addition to the rate of increase of heating,the final temperature must be great enough to nucleate boiling of thewater and decomposition of the GO oxides and intercalated ions. Thermalgravimetric studies indicate that temperatures of greater than 250° C.are required for complex vaporization of volatile components. If the GOis exposed to temperatures greater than 3000° C. excessive degradationof the GO structure may occur. However, that is the temperatureexperienced by the GO. GO samples exfoliated in flame burners mayinvolve flame temperatures in excess of 3000° C., but short residencetimes in the flames or the cooling effects of vaporization of solventsor evolved gases may keep the temperature experienced by the particleless than 3000° C., even though the flame temperature is greater.

The TEGO increases the conductivity of polymeric matrices by factors of10¹¹ to 10¹⁸ over the range of filler loadings between 0.1 to 20 wt %,preferably 1.5 and 5 wt %, based on the weight of the polymer compositeor ink formulation. The amount of filler includes all values andsubvalues there between, especially including 0.5, 1, 1.5, 2, 2.5, 3,3.5, 4 and 4.5 wt %. This corresponds to conductivity increases from10⁻¹⁹ S/m to 10⁻⁸-10⁻¹ S/m for a 1.5 to 5 wt % loading of TEGO in PMMA.Higher conductivities above 0.01 to 1000 S/n can be attainable in morehighly filled composite or ink formulations. The basic conductivity ofthe individual TEGO sheet is on the order of ½ to 1/10 of theconductivity of graphite based on the percentage of oxygens that disruptthe pure sp² graphitic structure. Commonly reported values for thein-plane conductivity of pure graphite sheets are 2 to 5×10⁵ S/m.

Polymers in which TEGO can be dispersed include, but are not limited to:polyethylene, polypropylene and copolymers thereof, polyesters, nylons,polystyrenes, polycarbonates, polycaprolactones, polycaprolactams,fluorinated ethylenes, polyvinyl acetate and its copolymers, polyvinylchloride, polymethylmethacrylate and acrylate copolymers, high impactpolystyrene, styrenic sheet molding compounds, polycaprolactones,polycaprolactams, fluorinated ethylenes, styrene acrylonitriles,polyimides, epoxys, and polyurethanes. Elastomers that can be compoundedwith TEGO include, but are not limited to, poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/di(propylene glycol)/polycaprolactone,poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/polytetrahydrofuran, amine terminatedpolybutadiene such as HYCAR ATB2000X173, carboxyl terminatedpolybutadiene such as HYCAR CTB2000X162, polybutadiene, dicarboxyterminated butyl rubber, styrene/butadiene copolymers, polyisoprene,poly(styrene-co-butadiene), polydimethysiloxane, and natural latexrubber. The polymers may be use alone or in combination.

It is possible to compound TEGO into the monomeric precursors of thesepolymers and to effect the polymerization in the presence of the TEGOnanofiller. The polymers and/or their precursors may be use alone or incombination.

Polar solvents into which TEGO can be dispersed include water,n-methylpyrolidone (NMP), dimethyformamide (DMF), tetrahydrofuran (THF),alcohols, glycols such as ethylene glycol, propylene glycol and butyleneglycol, aliphatic and aromatic esters, phthalates such as dibutylphthalate, chlorinated solvents such as methylene chloride, aceticesters, aldehydes, glycol ethers, propionic esters. Representativesolvents of the desired classes can be found at the Dow Chemical website (http://www.dow.com/oxvsolvents/prod/index.htm). The polar solventmay be used alone or in combination. Mixtures with non-polar solventsare possible.

The hydroxyl groups on the TEGO surface can be initiation sites fromwhich polymer chains can be grown using controlled free radicalpolymerization (RAFT, ATR, NMP, or MADIX polymerization) schemes. Anymonomer having a polymerizable can be used. Preferred monomers arearomatic monomers such as styrene, methacrylates, acrylates, butadienesand their derivatives. The monomers may be used alone or in mixtures.

The present invention relates to a thermally exfoliated graphite oxide(TEGO) produced by a process which comprises: (a) oxidizing and/orintercalating a graphite sample, resulting in a graphite oxide withexpanded interlayers; and (b) heating the graphite oxide to causesuperheating and gas evolution from the intercalated water and/orsolvent, the intercalant, and the decomposition of the graphite oxide.The rapid increase in pressure substantially exfoliates or disorders theGO layer stacking.

Substantial exfoliation of TEGO is defined by the absence of a X-raydiffraction peak from the original graphite peak at 2Θ˜26.5° (0.34 nmseparation distance between the graphene sheets), as shown by comparingthe XRD pattern in FIG. 4a for TEGO and the original XRD pattern forpure graphite in FIG. 1. There is less than 1% peak area in the range of2θ between 24 and 29° relative to the area of the broad TEGO peakbetween 2θ of 10-20°. Improper or incomplete exfoliation can result inmaterials shown in FIG. 4b which show the presence of the graphite peakand the broad TEGO peak. This material is not the material we refer toin this patent as TEGO. For the TEGO material described in the presentinvention, the area under the diffraction peak between 2θ=12.5 and14.5°, which is from the original GO sheet (see FIG. 4a ), is less thanis less than 15% of the total area under the TEGO peak between 2θ=9 and21°.

The present invention further relates to a method for manufacturing TEGOwhich comprises the steps noted above. The heating in step b) may takeplace in a furnace at a temperature of from 300 to 2000° C., preferably,800 to 1200° C. and most preferably at about 1000° C. The temperatureincludes all values and subvalues there between, especially including400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600,1700, 1800, and 1900° C. The higher the temperature, the shorter theheating time. The heating time also depends on the volume of the sampleand on any limitations heat conduction may pose. A sample having alarger volume may require a longer heating time. The heating time ispreferably between 1 sec and 5 min. The heating time includes all valuesand subvalues there between, especially including 5, 10, 20, 30, 40, 50,seconds, 1 min, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 minutes.

In another embodiment, step b) may take place by spraying through aflame at a temperature of about 2500° C. The transit time in this caseis in the order of a fraction of a second to about 1 second. Thesuperheating in step b) refers to the local hating of the water betweenthe sheet to a temperature of more than 100° C.

In a preferred embodiment, the process further comprises the steps ofremoving acids and salts from the graphene interlayers prior to heatingthe graphite oxide, as well as drying the graphite oxide to removeexcess water and solvent, while leaving intercalated species, adequatewater and solvent for exfoliation, prior to heating the graphite oxide.The salts being removed are the ionic species involved in the initialoxidation and intercalation. They include H⁺, K⁺, chlorate ions, nitrateions, sulfate ions, and organic acids that may arise from decompositionof the graphite structure.

In the context of the present invention, the phrase adequate waterrefers to the following. During heating to produce exfoliated TEGO thesuperficial water that is water on the surfaces of the oxidized GOsheets must be removed. This can be done in a “predrying” step to reducethe water content to between 500 wt % to 0.5 wt % (weight of water toweight of dry GO). The preferred water content for processes thatinvolve heating GO granular powders is between 75% and 2% water, and themost preferred range is 20% to 5%. These powders are subsequently heatedto induce exfoliation in a furnace, flame, fluidized bed, or microwaveheating device. Heating may also occur in a larger tube or by a flameprocess one could spray in an aqueous slurry of the GO. In the flameprocess the excess (superficial) water would vaporize without causingexfoliation. During the evaporation of superficial water, thevaporization keeps the temperature around the boiling point of thesolvent (i.e. ca 100° C.). Once the superficial water is evaporated,then the partially dried GO experiences the very high temperature andexfoliates.

Other processes for heating GO to rapidly expand it to TEGO may involveinjecting slurries of GO in bulk aqueous solution into the heatingdevice. These slurries may contain GO concentrations from 1-85 wt % GObased on the total weight of the slurry. The amount of GO includes allvalues and subvalues there between, especially including 5, 10, 15, 20,25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, and 80 wt. %. The slurriesmay be directly injected into a furnace which may be a tube furnace, afluidized bed heater, a flame burner with a reducing zone, or amicrowave chamber. The superficial water or solvent is initiallyevaporated and subsequently the GO with intercalated aqueous solvent issuperheated and the GO is exfoliated.

The TEGO produced in accordance with the present invention preferablyhas a surface area of from about 300 m²/g to 2600 m²/g, preferably 300m²/g to 2400 m²/g, more preferably 300 to 1100 m²/g, a bulk density offrom about 40 kg/m³ to 0.1 kg/m³ and a C/O oxygen ratio, after hightemperature expansion, in the range of from about 60/40 to 95/5, with arange of about 65/35 to 85/15 particularly preferred. The maximumcalculated surface area will be 2600 m²/g. based on the surface area ofa single graphite sheet. The surface area includes all values andsubvalues there between, especially including 400, 500, 600, 700, 800,900, 100, 110, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000,2100, 2200, 2300, 2400, and 2500 m²/g. The bulk density includes allvalues and subvalues there between, especially including 0.5, 1, 5, 10,15, 20, 25, 30, 35 kg/m³. The C/O oxygen ratio includes all values andsubvalues there between, especially including 65/35, 70/30, 75/25,80/20, 85/15 and 90/10. High temperature expansion occurs in thetemperature range of 250° C. or more, preferably at temperatures of from250 to 3000° C.

The TEGO of the present invention displays essentially no signature ofthe original graphite and/or graphite oxide as determined by XRD, and isproduced by a process that involves oxidation of layered graphite toproduce graphite oxide, using a material selected from e.g., sulfuricacid, nitric acid, hydrogen peroxide, perchlorate, or hydrochloric acidas oxidizers. The oxidant is not particularly limited. Preferredoxidants include KClO₄, HNO₃+KClO₃, KMNO₄+NaNO₃, K₂S₂O₈+P₂O₅+KMNO₄,KMNO₄+HNO₃, HNO₃. Another preferred method is polarization at a graphiteelectrode by electrochemical oxidation. Mixtures or combinations ofthese oxidants may be used. The resulting thermally exfoliated graphiteoxide functions as a nanofiller. The TEGO material displays essentiallyno signature of the original GO stacking as determined by XRD. Theheight of the X-ray peak between 2θ=10-15° is less than 20% of theheight of the peak between 2θ=22-30° in the original GO material whenX-ray measurements are calibrated for absolute scattering intensities.For improvement of mechanical properties, electrical and thermalconductivity of polymer composites, the aspect ratio of the nanofillershould be greater than 100, the filler should be of a size such that itsminor dimension is comparable to the dimensions of the polymer chains,and the filler should be uniformly dispersed in the polymer network.

The thermally exfoliated graphite oxide (TEGO) of the present inventionshows no visible sign of the 002 peak (either at 0.34 nm or 0.71 nminterplane separation distance) that characterizes graphitic materialsneither in the XRD nor in the SAED patterns. In a preferred embodimentof the present invention, there are several steps involved in thepreparation of TEGO: First is the complete intercalation and oxidationof graphite. This is needed so as to permit disruption of the London-vander Waals forces and to allow the incorporation of water or othervolatile solvent molecules into the stack structure. The acids and saltsare then removed from the graphene interlayers. The GO is thenappropriately dried to remove excess water or solvent, while leavingadequate solvents and intercalants to effect exfoliation. The dryingmethod is not particularly limited. Drying may take place at roomtemperature, at a temperature of from room temperature to 100° C., or ina vacuum oven. The GO is dried until the water or other solvent contentis between 1 and 500% by weight, preferably, 1 to 300% by weight andmost preferably 1 to 20% by weight, based on the total weight of the GO.The amount of water or other solvent includes all values and subvaluesthere between, especially including 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,12, 13, 14, 15, 16, 17, 18 19, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150,200, 250, 300, 350, 400, and 450% by weight. Finally, the GO is rapidlyheated to cause superheating of the intercalated water and thedecomposition of the intercalants. This causes the intercalated waterand the intercalants to vaporize or decompose faster than they candiffuse out of the interlayer spaces, generating large local pressuresthat force the graphite oxide layers apart. The result is the highlyexpanded TEGO structure with unique properties as a nanofiller.

The polarity of the TEGO surface can be modified to adjust thedispersion of the TEGO in liquid or polymeric matrices. Thismodification can be accomplished during processing by controlled theextent of reduction during exfoliation. This is accomplished bycontrolling the time and temperature history of the sample. After theinitial exfoliation leaving the sample at an elevated temperature willresult in less polar functionality. Exfoliation in an atmosphere withgas compositions favoring reduction will enhance reduction (such as COor CH₄), and gas compositions with higher oxidative power will enhancepolar functionality (such as mixed inert and oxygen gases). It ispossible to alter the polarity of the TEGO surface after production bychemical reaction through the OH, epoxide, and carboxylate groups on theTEGO surface.

In spite of nearly 150 years of extensive research on graphiteintercalation and expansion, complete exfoliation of graphite down toindividual graphene sheets has not been achieved. Thus far, thermal orchemical expansion and exfoliation of graphite have only producedmaterials with surface areas <600 m²/g, well below the theoretical valueof ˜2,600 m²/g predicted for completely delaminated graphene sheets.

The rapid thermal expansion of GO of the present invention offers aunique opportunity for very thin nanoplates to be used as a nanoscalereinforcer in polymer matrices. Due to the presence of polar oxygenfunctional groups on the surface of what the present invention refers toas TEGO, a polymer with polar or potentially reactive side groupsreinforced with TEGO has superior properties in comparison to similarlyprocessed nanocomposites containing single-wall carbon nanotubes(SWCNTs) and traditional EG.

TEGO may be used in polymer composites, particularly in conductivepolymer composites, as additive in elastomeric materials, in elastomerdiffusion barriers, as hydrogen storage medium, as material forsupercapacitors, in flexible electrodes, as adsorbent material, asdispersant, as lubricant, in coatings, particularly in coatings thatrequire UV stability. Further TEGO can be used in glass or ceramiccomposites, in thermoelectric composite materials, as pigments in inks,or as UV protective filler in composites. TEGO can also be used forelectromagnetic shielding, and oil spill remediation.

TEGO nanofillers can be added to polymer matrices to prepare polymercomposites. The large aspect ratio of the nano-sheets and the very highsurface area interfacing with the polymer matrix will produce compositeswith enhanced mechanical properties. Simulations (Gusev et al.Macromolecules 34 (2001) 3081) show that fillers with aspect ratiosgreater than 100 increase the tensile modulus at loading levels as lowas 3%. Work on surface-modified clay nanosheets has shown enhancement inmechanical properties. However, the dielectric mismatch between theorganic carbon matrix and the clay sheet has created problems indispersion of clays in composites. Further, the elastic modulus ofgraphene sheets vs. clays provides an added advantage in tuning theelastic properties of the composites to higher stiffness values. Theorganic composition of TEGO and its surface functionality allows itsincorporation into composites without extensive surfacefunctionalization and with facile dispersion. Polymers that can becompounded with TEGO nanofillers include, but are not limited to:polyethylene, polypropylene and copolymers thereof, polyesters, nylons,polystyrenes, polycarbonates, polycaprolactones, polycaprolactams,fluorinated ethylenes, polyvinyl acetate and its copolymers, polyvinylchloride, polymethylmethacrylate and acrylate copolymers, high impactpolystyrene, styrenic sheet molding compounds, polycaprolactones,polycaprolactams, fluorinated ethylenes, styrene acrylonitriles,polyimides, epoxys, and polyurethanes. Elastomers that can be compoundedwith TEGO include, but are not limited to, poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/di(propylene glycol)/polycaprolactone,poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/polytetrahydrofuran, amine terminatedpolybutadiene such as HYCAR ATB2000X173, carboxyl terminatedpolybutadiene such as HYCAR CTB2000X162, polybutadiene, dicarboxyterminated butyl rubber, styrene/butadiene copolymers, polyisoprene,poly(styrene-co-butadiene), polydimethysiloxane, and natural latexrubber. TEGO-polymer composites can be applied as building materialreinforcements, wire coatings, automotive components (including bodypanels) etc.

The conductivity imparted by the conductive TEGO filler at low loadinglevels enables the preparation of conductive composites. The advantageof conductivity at low loadings is that the mechanical, and especiallythe fracture, properties of the composite are not compromised. Theamount of TEGO in the polymer composite is 0.1 to 90%, preferably 1 to80%, more preferably 5-50% by weight based on the total weight of thecomposite. Another preferred range is 0.1 to 5%, preferably 0.5 to 2% byweight based on the total weight of the composite. The conductivepolymer composites find great utility in the area of electrostatic spraypainting of polymer parts. The low levels of conductivity imparted bythe TEGO allow dissipation of the charge from the charged aerosol drops.Electrostatic spraying eliminates “overspray” (i.e. spray that missesthe target) and minimizes environmental hazards associated with aerosolsprays and solvents. The conductivity of TEGO also enables applicationsof electrical shielding, such as for computer housings. It can be usedfor making thermal overload protective devises wherein heat or excesscurrent flow through the conductive composites causes an expansion ofthe matrix and a drop in conductivity as the TEGO sheets no longerpercolate. The level of conductivity and decrease in conductivity uponheating can be tailored to make either current-limiting devices orthermal switches. Very conductive TEGO-polymer composites can be used asconductive inks and for making conductive circuitry. The lines orconductive features can be patterned by application of apolymer-TEGO-solvent fluid with subsequent drying. Polymers which can beemployed in the production of conductive composites include, but are notlimited to: polyethylene, polypropylene and copolymers thereof,polyesters, nylons, polystyrenes, polyvinyl acetates and its copolymers,polycarbonates, polyvinyl chloride, polymethylmethacrylate and acrylatecopolymers, polycaprolactones, polycaprolactams, fluorinated ethylenes,high impact polystyrene, styrenic sheet molding compounds, styreneacrylonitriles, polyimides, epoxys, and polyurethanes. Elastomers thatcan be compounded with TEGO include, but are not limited to,poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/di(propylene glycol)/polycaprolactone,poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/polytetrahydrofuran, amine terminatedpolybutadiene such as HYCAR ATB2000X173, carboxyl terminatedpolybutadiene such as HYCAR CTB2000X162, polybutadiene, butyl rubber,dicarboxy terminated styrene/butadiene copolymers, polyisoprene,poly(styrene-co-butadiene), polydimethysiloxane, and natural latexrubber.

Currently, carbon blacks are added to elastomers to impart desirablemechanical properties. Most importantly the carbon black creates amodulus that increases with strain. This non-linearity protects rubberfrom damage during large deformations. The TEGO filler will providesimilar enhanced non-linear strain hardening to elastomers. Theinterface is similar to that of carbon black, but the flexibility of theTEGO nano-sheet enables deformation at low strains and hardening athigher deformations. The TEGO is superior to other clay nano-plateletsthat have been considered for these applications for two reasons: (1)the carbon structure of TEGO has better interfacial compatibility withelastomeric matrices than do inorganic clay sheets, and (2) the greaterflexibility of the TEGO sheet, compared to clays, decreases interfacialfatigue and debonding. Polymers that can be compounded to produceelastomers with enhanced modulus and toughness include, but are notlimited to, include, but are not limited to,poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/di(propylene glycol)/polycaprolactone,poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/polytetrahydrofuran, amine terminatedpolybutadiene such as HYCAR ATB2000X173, carboxyl terminatedpolybutadiene such as HYCAR CTB2000X162, butyl rubber, polybutadiene,dicarboxy terminated styrene/butadiene copolymers, polyisoprene,poly(styrene-co-butadiene), polydimethysiloxane, and natural latexrubber.

Butyl rubber has excellent gas diffusion barrier properties and is,therefore, used as the lining for tubeless tires and for inner tubes.However it is significantly more expensive than other elastomers.Rubbers and elastomers that are used in tire applications do not havesufficient gas diffusion barrier properties to function in tireapplications without the butyl rubber lining layer. TEGO nano plateletswith aspect ratios between 1000 and 10,000 can provide excellent barrierproperties when added to conventional rubbers and elastomers andoriented perpendicular to the direction of gas diffusion. Barrierproperties of up to 1000 times greater than that of the unfilled rubberare possible. Elastomers that can be compounded to produce materialswith enhanced barrier properties include, but are not limited to,poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/di(propylene glycol)/polycaprolactone,poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/polytetrahydrofuran, amine terminatedpolybutadiene such as HYCAR ATB2000X173, carboxyl terminatedpolybutadiene such as HYCAR CTB2000X162, butyl rubber, polybutadiene,dicarboxy terminated styrene/butadiene copolymers, polyisoprene,poly(styrene-co-butadiene), polydimethysiloxane, and natural latexrubber.

TEGO added to polymer films, packaging materials, flexible tubing formedical applications, suits for chemical and biological warfare, glovesfor chemical protection and other applications required enhanced barrierproperties are also achievable. Also, the metal liners used as gasdiffusion barriers in glass or carbon fiber wrapped high-pressure gasstorage cylinders add extra weight and reduce the cycle-life of thecylinders. TEGO filled gas diffusion barrier composites can be used toin place of the metal liners to improve the performance of high-pressuregas storage cylinders.

There is significant interest in materials for hydrogen storage. TEGOhas three unique characteristics that make it attractive as a hydrogenstorage medium that will operate at more moderate pressures andtemperatures than conventional materials or carbon nano tubes. (1) Theability to covalently “stitch” TEGO or graphite oxide layers usingdivalent chains allows the preparation of TEGO or graphite oxide sheetswith interlayer spacings of approximately 1-1.4 nm. This is thepredicted spacing that maximizes hydrogen storage between graphitesheets. Stitching can be accomplished, for example, with alkyl diaminesreacting with the surface epoxides on the TEGO surfaces. The interlayerspacing is determined by the alkyl chain length. (2) The Stone-Walesdefects introduced to the graphene sheet by oxidation provide enhancedhydrogen binding relative to binding to pure graphite sheets. (3) Thepolar functionality on TEGO can be used to localize metal clusters onthe surface that act to dissociate diatomic hydrogen into molecularhydrogen and increase the rate of saturating and emptying the TEGOnano-sheet. This phenomenon is called “spillover” in the hydrogenstorage literature. Only TEGO and graphite oxide have these multiplecharacteristics that make them effective hydrogen storage materials.

Supercapacitors are playing a significantly important role in hybridenergy sources. The material of choice in all commercial supercapacitorsis high surface area carbon either as carbon aerogel or expandedgraphite. TEGO provides an advantage over both materials in due to itshigher surface area and planar structures.

The ability to make conductive TEGO dispersions and pastes, as well asconductive polymer composites opens the door for applications aselectrodes for batteries, sensors, and electronic devices. The relativeinertness of the TEGO graphitic sheet, coupled with its deformabilitymakes it an attractive candidate for electrode applications. The planarstructure of TEGO makes it an attractive material to make very thinelectrodes for flat surface applications.

The high surface area of TEGO and the layered structure that is possibleto achieve make it an attractive adsorbent material to compete withactivated carbon. The gallery size between layers can be tailored by“stitching” (described above) to produce samples with interlayerspacings between 7.1 nm and 15 nm. Therefore, the adsorption can betailored to optimize the binding of species with specific sizes. Thissize selectivity, polar sites on the TEGO surface, the ability tofunctionalize the TEGO surface, enable the production of adsorbents withunique size selectivity and chemical specificity. The size specificityis shown between molecules over a range of 1 to 50 nm, preferably 1-20nm. The size includes all values and subvalues there between, especiallyincluding 5, 10, 15, 20, 25, 30, 35, 40 and 45 nm. It is especiallyuseful in the separations of proteins.

Current absorbents and absorptive media for protein and DNA fragmentseparations are often based on silica or cellulose particulates in thesize range of 10-1000 microns. The substrates provide mechanical supportand reactive groups that can be used to couple ligands and functionalgroups to the particle surfaces. A disadvantage of the silica-basedmedia is the relative instability of the particles and surface linkagesat pH's above 8. The disadvantage of the cellulose-based supports is therelative difficulty in conjugating ligands and functionality to thehydroxyls on the cellulose surfaces.

The TEGO material combines the advantages of high surface area andreadily functionalizable epoxide and carboxyl groups on the TEGOsurfaces. In this invention the surface of the TEGO is made anionic byreaction of carboxylic acid and/or sulfonic acid containing reactantswith amine functionality. The facile reaction with the TEGO epoxidesunder mild conditions of reflux conditions in ethanol enable surfacemodification. To provide anionic surfaces. Reaction with diaminesprovides amine surface functionality that can be further quaternized tocreate permanent cationic charge. Functionalization using reactionscommonly employed to functionalize cellulose media can be used tofunctionalize through the TEGO surface hydroxides. Once the surface isfunctionalized with the ion exchange moiety or an affinity tag ligand,the surface can be further functionalized with PEG or dextran functionalreagents to passivate the surface to make it resistant to proteinadsorption or denaturation. The TEGO, thus functionalized can be used asa bulk filling for chromatography columns or can be compressed oragglomerated to make a macro-particulate media in the size range 10-5000microns that can be used as a chromatography packing.

The native and functionalized TEGO can also be used as an adsorptivemedia for gas phase separations. The functionalized TEGO described abovecan be directly used as packings for gas chromatography applications.

The unique blend of hydrophilicity and hydrophobicity that arise fromthe polar and non-polar groups on the TEGO surface and its largeplatelet size make it an effective dispersant for oil in water and waterin oil emulsions. Oils include alkanes, aromatic hydrocarbons,chlorinated hydrocarbons, heterocyclics, petroleum distillates rangingfrom light hydrocarbons (C4-C8), to heavy vacuum residuals (C18-C40+),natural oils such as corn, safflower, linseed, olive, grape seed,silicone fluids and oils, fatty acids and fatty acid esters. Thepolarity of the TEGO can be tuned by the exfoliation conditions. Thedegree of reduction during the high temperature treatment determines thebalance of oxidized surface groups (polar) to reduced graphitic sites(nonpolar). Further, post reaction through the surface epoxides, amines,and hydroxyls can be used to further tune or modify polarity. Thematerials are especially effective at dispersing crude oil in wateremulsions that are being used as drilling fluids in oil and gasoperations, and as mobility control agents in the recovery of oil fromtar sands (Canadian patent Exxon Chemical 2067177). They are especiallypreferred for emulsification of tars and asphaltenes in applicationssuch as paving compounds and sealing compounds.

Graphite is an excellent lubricant especially in high temperatureapplications due easy sliding of graphene sheets over each other. Weexpect TEGO to display superior lubricating properties since theinteractions between the graphene sheets are significantly weakened incomparison to graphite.

The UV light absorption capabilities of TEGO make it an attractiveadditive to coatings that must maintain stability exposed to sunlight.Coatings include preferably black coatings. TEGO can be used as anadditive for roofing sealers, caulks, elastomeric roofing membranes, andadhesives.

TEGO absorbs UV radiation and can therefore be used to impart UVprotection and to improve the lifetime of plastic components in outdooruse, such as hoses, wire coatings, plastic pipe and tubing etc.

TEGO can be added to a ceramic matrix to improve the electricalconductivity and the fracture toughness of the material. The partiallyoxidized surface of TEGO offers stronger interaction with the ceramicmatrix, especially with metal oxides and silicon oxides in particular.For example, TEGO can be mixed with a silicon alkoxide material and thenthe silicon alkoxide can be condensed to form an amorphous materialsilicon oxide material containing well-dispersed TEGO nano-platelets.The hydroxyl and epoxide groups on the TEGO surface can condense withthe silicon alkoxide to form strong covalent bonds between the matrixand the TEGO filler. Low loadings of TEGO in such materials impartimproved fracture strength and conductivity. TEGO-glass and TEGO-ceramiccomposites can also be applied as thermoelectric materials. Similartechniques can also be used to create tinted and UV-protective grades ofglass. TEGO can also be used to reinforce cement and in other buildingmaterial applications.

Due to the very low loadings of TEGO required to impart electricalconductivity to a non-conductive matrix, TEGO can form compositematerials with greatly enhanced electrical conductivities but withthermal conductivities approximately the same as those of the matrixmaterials. This combination leads to TEGO-composites with improvedthermoelectric figures of merit. The matrix material for thisapplication can be either organic or inorganic, with excellentthermoelectric properties expected from the TEGO-silica composites, asnoted above. The electrical conductivity of and nature of the carrier(i.e. electrons versus holes) in the material can be tailored byaltering the surface chemistry of the TEGO filler or by modifications tothe matrix material.

Carbon black and other carbon materials are frequently used as a pigmentin inks. The very small size of the TEGO nano-platelets can lead to anink with an exceptionally high gloss (i.e. low surface roughness of thedried ink). The surface chemistry of TEGO can also be easily modified toproduce different colors, tones and tints.

The conductive properties of TEGO enable its use in electromagneticshielding. Applications such as the enclosures for computer housings,computer screens, electronic devices such as medical diagnostics, andconsumer electronics often require screening so that electromagneticsignals are either contained in the device and do not escape to provideinterference for other devices, or to prevent external fields frominterfering with the electronic components inside the enclosure.Currently conductive carbon black fillers are often used in theseapplications or conductive expanded graphite fillers. The TEGOconductive fillers can be used in these applications at lower loadinglevels and with less deleterious impact on the mechanical properties ofthe polymer matrices. In addition to the TEGO being added to thestructural polymer used in these applications, the TEGO can beincorporated into a solvent phased system with binder to make aconductive paint that can be applied to the interior of the housing toprovide electromagnetic shielding.

Currently expanded graphite is used as an absorbent for oil spillremediation and for the cleanup of other hazardous organic liquidspills. The hydrophobic surfaces are wetted by oil and thereby bind andhold oil. Other compounds used for spill remediation are clays, butthese must be surface treated to may them hydrophobic enough to bindorganic liquids. The high surface area of TEGO and its hydrocarbonsurfaces make it an excellent absorbent material for oil and organicliquids. The TEGO can be contained in large porous sacks made frompolypropylene or polyethylene fabric or porous film. The low bulkdensity of TEGO make it attractive in that the amount of liquid that canbe imbibed on a weight basis can be high. Liquid loadings between 100 to10,000 wt:wt oil to TEGO can be achieved. In another embodiment the TEGOis co-processed with a polymeric binder in the form of a foam sheet.These open cell structure of the foam allow contact between the oil andthe TEGO surfaces. The advantage of this system is that the absorbentsystem can be rolled for storage.

While the present invention shows a high surface area value for theexfoliated graphene by N₂ adsorption, this may not be the most relevantmeasure of the ability to disperse the graphene sheets, in, for example,a polymeric matrix. While adsorption measurements reflect porosity andsurface area of three dimensional structures and powders, graphenecomprises two-dimensional, flexible sheets. In the solid dry state thegraphene sheets must be in contact, and the contact areas will occludenitrogen intrusion in the adsorption measurement. A more appropriateanalogy for graphene may be to consider it as a two-dimensional polymer.An important question for applications involving graphene in polymermatrices is the degree of dispersion, or the effective surface area, inthe dispersed state. The present invention TEGO materials dispersereadily in polar organic solvents such as THF to form a uniformdispersion.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only, and are not intended to belimiting unless otherwise specified.

EXAMPLES

Materials and Methods:

SWCNTs (BuckyPearls, lot no. CTU3-2005B-2) from Carbon Nanotechnologies,Inc. (Texas, USA); PMMA (Mw=350000, PDI=2.7) from Polysciences(Warrington, Pa., USA); and organic solvents, all HPLC grade, fromFisher Scientifics (Hanover Park, Ill., USA) were used as received.Flake 1 Graphite was from Asbury Carbon Co. (Asbury, N.J., USA).

Preparation of Graphite Oxide (GO):

Graphite oxide was prepared from Flake 1 graphite according to themethod of Staudenmaier (L. Staudenmaier, Ber. Dtsh. Chem. Ges, 31, 1481,(1898)). Graphite (1 g) was added to a 500-ml round-bottom flaskcontaining a stirred and cooled (0° C.) mixture of concentrated sulfuricand nitric acid (2:1 v/v, 27 ml). Potassium chlorate (11 g) was thenadded gradually in small portions to ensure that the temperature of thereaction mixture did not rise above 30° C. After the addition ofpotassium chlorate, the mixture was allowed to reach room temperatureand stirring was continued for 96 h. Next, the mixture was poured intodeionized water (1 l) and filtered over a 60-ml fritted funnel (coarse).The product was washed on the funnel with 5% aqueous HCl until sulfateswere no longer detected (when 5-ml of the aqueous filtrate does not turncloudy in the presence of one drop of saturated aqueous BaCl₂) and thenwith deionized water (2×50 ml). The resulting graphite oxide was driedin an oven at 100° C. for 24 h. Elemental analysis (Atlantic Microlab,Norcross, Ga.): C, 53.37%, O, 39.45%, H, 1.62%, N, 0.14%.

Preparation of Expanded Graphite (EG):

Flake 1 Graphite (1 g) was treated with 4:1 v/v mixture of concentratedsulfuric and nitric acid (50 ml) for 24 h at room temperature. Uponcompletion, the suspension was diluted with water (150 ml) and filtered.The solid residue was washed with copious amounts of water until thefiltrate was no longer acidic. The resulting material was dried in anoven at 100° C. overnight. Next, the dried material was placed in aquartz tube and the tube heated rapidly with a propane blow torch (ModelTX9, BernzOmatic, Medina, N.Y.) set at medium intensity while underdynamic vacuum to produce the expanded graphite (FIG. 7).

Preparation of TEGO by Method A:

Graphite oxide (0.2 g) was placed in an alumina boat and inserted into a25-mm ID, 1.3-m long quartz tube that was sealed at one end. The otherend of the quartz tube was closed using rubber stopper. An argon (Ar)inlet and thermocouple were then inserted through the rubber stopper.The sample was flushed with Ar for 10 min, then the quartz tube wasquickly inserted into a Lindberg tube furnace preheated to 1050° C. andheld in the furnace for 30 s. Elemental analysis of a sample oxidizedfor 96 h indicates a C/H/O ratio of 54/25/21 (by mol) while theelemental analysis of TEGO shows an increase in C/O ratio from 6/4 in GOto 8/2.

Preparation of TEGO by Method B:

Graphite oxide (0.2 g) was placed in a 75-ml quartz tube equipped with a14/20 ground glass joint. The tube was evacuated and backfilled withnitrogen three times and then attached to a nitrogen bubbler. Next, theGO was heated rapidly with a propane blow torch (Model TX9, BernzOmatic,Medina N.Y.) set at medium intensity until no further expansion ofgraphite oxide was observed (typically 15 s.). Elemental Analysis: C,80.10%, O, 13.86%, H, 0.45%, N, 0%.

Dispersion of TEGO in Organic Solvents:

The dispersions of TEGO were made at 0.25 mg/ml concentration bysonicating TEGO (prepared by method B, 5 mg) in a given solvent (20 ml)for 5 h in a Fisher Scientific FS6 ultrasonic bath cleaner (40 wattpower). The dispersions were then left standing under ambientconditions.

The following was observed: TEGO dispersions in methylene chloride,dioxane, DMSO and propylene carbonate precipitated within 8 h aftersonication. The dispersion in nitrobenzene was more stable, but after 24h the precipitation of TEGO was complete. In THF, a moderately stabledispersion was observed accompanied by fairly substantial precipitationafter 24 h. However, the THF dispersion still remained blackish after aweek. More stable dispersions can be obtained in DMF, NMP,1,2-dichlorobenzene, and nitromethane: they were still quite black afterone week albeit with a small amount of sedimentation.

Experimental Procedure for the AFM Imaging:

The AFM images were taken on an AutoProbe CP/MT Scanning ProbeMicroscope (MultiTask), Veeco Instruments. TEGO was dispersed in1,2-dichlorobenzene by sonication (vide supra) and the dispersiondeposited on a freshly cleaved mica surface. Imaging was done innon-contact mode using a V-shape “Ultralever” probe B (Park ScientificInstruments, B-doped Si with frequency f_(c)=78.6 kHz, spring constantsk=2.0-3.8 N/m, and nominal tip radius r=10 nm). All images werecollected under ambient conditions at 50% relative humidity and 23° C.with and a scanning raster rate of 1 Hz. The AFM image in FIG. 8 showsstacks of TEGO nanostack with thickness of ˜2 nm.

X-Ray Photoelectron Spectroscopy (XPS):

XPS measurements were performed using an Omicron ESCA Probe (OmicronNanotechnology, Taunusstein, Germany) located at NorthwesternUniversity's Keck Interdisciplinary Surface Science Center withmonochromated Al K_(α) radiation (h v=1486.6 eV). The sample wasoriented with a 450 photoelectron take-off angle from the sample surfaceto the hemispherical analyzer. Data were collected using a 15-kVacceleration voltage at 20-mA power and vacuum of 10⁻⁹ mbar. An analyzerpass-energy of 50 eV with 500-meV steps was used for single-sweep surveyscans. High-resolution spectra were averaged over three sweeps using ananalyzer pass energy of 22 eV with 20-meV steps. TEGO samples werede-gassed overnight within the XPS chamber (10⁻³ mbar) prior toanalysis. The raw C_(1s) XPS data (FIG. 9) were analyzed using multipakand XPS peak 41 fitting software to determine the relative peaklocations and areas in relation to standard binding energies for knowncarbon functionalities (Handbook of X-ray photoelectron spectroscopy,edited by J. Chestain, R. C. King Jr., Physical Electronic, Inc., EdenPrairie, USA (1992)). The main component at 284.6 eV is attributed to Cin C—C bond. An additional component at 286.1 eV is attributed to C in—C—O— or C—O—C moieties.

The atomic concentration was calculated from the relation (Surfaceanalysis method in materials science, edited by D. J. O'Connor, B. A.Sexton, R. St. C. Smart, Springer-Verlag, Heidelberg, (1992)):C=(I_(i)/S_(i))/Σ_(i)(I_(i)/S_(i)), where I_(i) is the peak intensityfor element i and S_(i) is the sensitive factor for the peak i.Sensitive factor for C_(1s) is 1 and O_(1s) is 2.85. From peak intensityintegration, the oxygen concentration is calculated to be 8.4 atomic %.

Processing of Nanocomposites:

The TEGO used for nanocomposite was prepared via both methods A and B.Consistent composite properties were obtained regardless of the methodof TEGO preparation. Depending on the wt % of the composite, each typeof nano-filler was initially dispersed in tetrahydrofuran (THF, 10 ml)by bath sonication (Branson 3510, 335 W power) at room temperature.These solutions were then combined with a solution of PMMA in THF (10-30ml). Shear mixing (Silverson, Silverson Machines, Inc., MA. USA) at 6000rpm was then applied to the TEGO/PMMA and EG/PMMA systems for 60 min inice-bath to reduce the frictional heat produced in polymer by the shearmixer while the SWNT/PMMA systems received additional bath sonicationfor 60 min (shear mixing was not used for SWNT/PMMA). The compositesolutions were then coagulated with methanol, filtered under vacuumusing polycarbonate filter paper (Millipore, Cat. No. TCTP04700; 10-μmpore size), and dried at 80° C. for 10 h to yield a solid flakymaterials. Nano-filler/PMMA composite samples for mechanical testingwere pressed into a thin film between stainless steel plates using0.1-mm thick spacers in a Tetrahedron (San Diego, Calif.) hydraulichot-press at 210° C. under 2 MPa to approximately 0.12-0.15 mmthickness. A neat PMMA control sample was prepared in the same manner.

Mechanical Analysis:

The viscoelastic response of these composites was measured using DynamicMechanical Analysis (DMA 2980, TA instruments, DE, USA). Strips ofuniform width (6 mm) were cut from the film using a razor blade. Atensile force with 0.1-N pre-load was applied to the test specimen usinga film tension clamp and dynamic oscillatory loading at a frequency of 1Hz and amplitude of 0.02% strain was applied. Storage modulus (FIG. 10)and tan delta (FIG. 13) were obtained in temperature sweeps of 3°C./min. The stress-strain curves and ultimate strength of the compositeswere obtained according to ASTM D882 using Minimat (TA Instruments, DE,USA).

Thermal Property Measurements:

Thermal degradation properties of the composites were examined bythermal gravimetric analysis (TGA) on a TA Instruments SDT 2960Simultaneous DTA-TGA instrument. Pieces of the composites (˜10 mg) wereloaded into to the TGA instrument and heated from 40° to 800° C. at arate of 10° C. per minute under N₂ atmosphere. Data are shown in FIG.11.

Scanning Electron Microscopy (SEM):

SEM imaging was used to examine EG and TEGO morphology ex situ as wellthe fracture surfaces of the nanocomposites using a LEO 1525 SEM (LEOElectron Microscopy Inc, Oberkochen, Germany) (FIG. 12). Nanocompositesamples were mounted on a standard specimen holder using double-sidedcarbon conductive tape with the fracture surfaces toward the electronbeam. An acceleration voltage was varied between 1 kV-20 kV depending ondifferent imaging purposes and sample properties.

Glass Transition Temperature Measurements:

The glass transition temperature, T_(g) of each composite was obtainedfrom the tan delta peaks from the DMA experiment described in SI-5 (FIG.13). DMA results (normalized tan delta peak) are shown for allnanocomposites at 1 wt % loading as well as for TEGO/PMMA at two lowerwt % loadings. Results show a peak broadening but no shift in the T_(g)for SWNT/PMMA and a modest increase in T_(g) for EG/PMMA. TEGO/PMMAnanocomposite shows a rheological percolation even at the lowest wt %measured, 0.05%, with a nearly-constant T_(g) shift of 35° C. for all wt% measured.

AC Conductivity Measurements:

Composite samples were microwave plasma-etched (Plasma-Preen 11-862,Plasmatic Systems, N.J.) for 1 min at 2 Torr of O₂ and 350 W of power.AC impedance measurements were performed using an impedance analyzer(1260 Solartron, Hampshire, UK) with a 1296 Solarton dielectricinterface. The specimen was sandwiched between two copper electrodesthat are held tightly together with two 2-mm thick polycarbonate plates.Electrically conductive colloidal graphite (Product no. 16053, Ted PellaInc., Redding Calif., USA) was applied between the sample and copperelectrode to avoid point-contacts caused through surface roughness ofthe nano-composites. Impedance values were taken for the nanocompositesbetween 0.01-10⁶ Hz. Conductivity of the polymer nanocomposites (seeFIG. 15) is taken from the plateau at low frequencies at 0.1 Hz.

DC Conductivity Measurements:

Hot-pressed composite samples having thickness about 0.1 mm were cutinto strips that are 1-2 mm wide and 15-20 mm long. The strips weremicrowave plasma-etched (Plasma-Preen II-862, Plasmatic Systems, N.J.)for 1 min at 2 Torr of O₂ and 350 W of power. Subsequently, 25-nm thickgold films were thermally deposited on the specimen surfaces: four padson one side of the composite strip for the longitudinal measurements andone pad on two opposing sides of the strip for the transversemeasurements. Pad spacing for longitudinal measurement were always 0.16mm (determined by mask geometry during the deposition). Pad spacing fortransverse measurements were preset by the sample thickness. Copperwires were attached to these gold-platted pads using silver-filled epoxy(H₂OE, EPO-TEK, MA). Four-point probe DC-resistive measurements wereperformed using an HP multimeter (HP34401A). As a first approximation,the composite electrical resistivity was calculated from known specimengeometry. In these initial results, longitudinal and transverseresistivities diverged considerably, especially for EG-filledcomposites. Transverse resistivities were always higher thanlongitudinal ones. However longitudinal measurements, considering theelectrical leads configuration, include both longitudinal, I_(l) andtransverse, I_(l) components of the current path (FIG. 14). In order toseparate these two components, the current distribution across thespecimen was modeled based on the finite-element method (Femlab 3.1,Comsol AB). For each measured sample, we input actual specimen andelectrical pads geometry, transverse resistivity, and longitudinalresistance to obtain the computed longitudinal resistivities that arereported in this paper.

X-Ray Diffraction (XRD) Measurement

XRD patterns of graphite, GO, and TEGO are recorded in a Rigaku MiniFlexdiffractometer with Cu Kα radiation. Initial, final and step angles were5, 30 and 0.02° respectively.

Example 1

Graphite oxide was prepared from graphite by a process of oxidation andintercalation, referred to as the Staudenmaier method. The method uses acombination of oxidizers and intercalants: sulfuric acid, nitric acidand potassium chlorate under controlled temperature conditions. Ratiosof graphite to potassium chlorate in the range of 1:8 to 1:20 (wt/wt)are preferred. Ratios of sulfuric to nitric acid from 5:1 to 1:1 arepreferred. The Staudenmaier method is the preferred oxidation procedure.

In this example, 5 g graphite flake with a 400 μm average flake size(Asbury Carbon, Asbury, N.J.) was added to an ice-cooled solutioncontaining 85 ml sulfuric acid and 45 ml nitric acid. This was followedby the addition of 55 g potassium chlorate over 20 minutes such that thetemperature did not exceed 20° C. After this oxidation/intercalationprocess proceeded for 96 hours, the reaction mixture was poured into 7 lof deionized water and filtered using an aspirator. The oxidizedgraphite was then washed with 5% HCl until no sulfate ions were detectedin the filtrate, using the barium chloride test. The oxidized graphitewas then washed with DI water until the filtrate had a pH of 5 orgreater. The sample was examined by XRD to demonstrate completeoxidation by the elimination of the original sharp diffraction peak ofgraphite.

Example 2

In preparing thermally exfoliated graphite oxide (TEGO), graphite oxide(0.2 g) was placed in a ceramic boat and inserted into a 25 mm ID, 1.3 mlong quartz tube that was sealed at one end. The other end of the quartztube was closed using a rubber stopper. An argon (Ar) inlet andthermocouple were then inserted through the rubber stopper. The samplewas flushed with Ar for 10 minutes; the quartz tube was then quicklyinserted into a preheated Lindberg tube furnace and heated for 30seconds.

Example 3

XRD patterns of graphite, GO, and TEGO were recorded in a RigakuMiniFlex diffractometer with Cu Ku radiation. Initial, final and stepangles were 5, 30 and 0.02, respectively. The surface area of TEGO wasmeasured by nitrogen adsorption at 77K using a Micromeritics FlowSorbapparatus with a mixture of N₂ and He 30/70 by volume as the carriergas. High-resolution XPS spectra were obtained using an Omicron ESCAProbe (Germany). Samples were de-gassed overnight within the XPS chamber(10-3 mbar) prior to analysis of the sample. Data were collected using15 kV and 20 mA power at 10-9 mbar vacuum. The raw XPS data wereanalyzed to determine peak locations and areas in relation to specificbinding energies that best fit the experimental data. The main C—C peak(C_(1s)) at 284.6 eV was observed. An additional photoemission presentat higher binding energy peaks at 286.1 eV represented —C—O— or C—O—Cbonding.

Example 4

The solid-state magic angle spinning (MAS) 13C NMR spectrum of thegraphite oxide was obtained using a Chemagnetics CMX-II 200 spectrometerwith a carbon frequency of 50 MHz, a proton frequency of 200 MHz, and azirconia rotor of 7.5 mm diameter spinning at 4000 Hz. To enableseparation of the carbon peaks of the solid GO sample a so called,“Block pulse sequence” was used. This employs a decay pulse sequencewith a 450 pulse angle of 2.25 ms, high-power proton decoupling (˜50kHz), and a 20 s delay between pulses. The spectrum was run at roomtemperature and 5120 scans were acquired with 4 K data points each. Thechemical shifts were given in ppm from external reference of thehexamethylbenzene methyl peak at 17.4 ppm.

Example 5

FIG. 1 shows the XRD diffraction patterns of the graphite flakes afteroxidation for 48, 96, 120 and 240 hours. Note that as oxidationproceeds, a new peak characteristic of GO appears at a d-spacing ofabout 0.7 nm (2Θ=12.2°), and the intensity of the native graphite 002peak (2Θ=26.7°) decreases significantly. Note also that after oxidationfor 96 hours or longer, the graphite 002 peak essentially disappears. Atthis point, intercalation is achieved, as the graphene layers are nolonger about 0.34 nm apart (as they were initially), but are now about0.71 nm apart. The graphite oxide samples having d spacings of about0.71 nm correspond to about 12% adsorbed water.

Example 6

The selected area electron diffraction (SAED) pattern of the oxidized,but not exfoliated, sample is shown in FIG. 2. SAED patterns areobserved by focusing beam at a selected area to obtain electrondiffraction information on the structure of matter. The SAED was takenover a large area; therefore, it contains the information from many GOgrains. A typical sharp, polycrystalline ring pattern is obtained. Thefirst ring 21 originates from the (1100) plane, with the second ring 22arising from the (1120) plane. In addition, strong diffraction spotswere observed on the ring. The bright spots corresponding to the (1100)reflections within the ring retain the hexagonal symmetry of the [0001]diffraction pattern. It is therefore postulated that the GO sheets,before thermal treatment, are not randomly oriented with respect to oneanother, and the interlayered coherence is not destroyed at this stage.

Example 7

It is further postulated that GO contains aromatic regions composedentirely of sp² carbon bonds and aliphatic sp³ regions that containhydroxyl, epoxy, and carboxylic groups. Elemental analysis of a sampleoxidized for 96 hours indicates a C/H/O ratio of 54/25/21 (by mol). The¹³C-NMR spectrum for a sample oxidized for 96 hours is shown in FIG. 3.The spectrum contains three distinguishable peaks, at chemical shifts(δ) of about 60-70, 133, and 210-220 ppm. The peak between 60 and 70 ppmis anticipated to be composed of two peaks, which can be assigned tohydroxyl and epoxy groups. The peak at 133 ppm corresponds to aromaticcarbons, while the third peak at 210-220 ppm may be assigned to carbonattached to carbonyl oxygen.

Example 8

In an exemplary embodiment, in order to form TEGO, a graphite oxidesample that has been oxidized for 96 hours is heated under argon for 30seconds at different temperatures. It was found that heating theexpanded GO at 200° C. is sufficient for partial exfoliation. However,the extent of exfoliation increases as the temperature increases. Theexfoliation results in a large apparent volume expansion (about 200-400times the original volume). The TEGO prepared from completely oxidizedsamples has a fluffy “black ice-like” structure. FIGS. 4a and 4b showthe XRD spectrum of graphite, GO oxidized for 96 hours, and a TEGOsample prepared by rapid heating of the GO sample. TEGO samples show nosign of the 002 peak for either the graphite oxide (2Θ≈12.2°) or for thepristine graphite (2Θ≈26.5°). In contrast, heating a partially oxidizedsample yields an XRD diffraction pattern that contains the 002 peak ofthe pristine graphite, as shown in FIG. 4 b.

Example 9

Large area SAED patterns (FIG. 5) demonstrate enhanced exfoliation ofthe layers. The diffusion rings (51 and 52) are very weak and diffuse.These weak and diffuse diffraction rings, typically observed withdisordered or amorphous materials, suggest that the alignment betweenthe sheets and the long-range coherence along the c direction isessentially lost in the thermal exfoliation.

Due to the elimination of water and some oxygen functional groups duringthe rapid heating step, the structure of TEGO has a higher C/O ratiothan the parent GO. Elemental analysis shows an increase in the C/Oratio of from 6/4 in GO to 8/2 in TEGO.

The surface area of TEGO samples prepared from a GO sample that wasoxidized for 120 hours and heated for 30 seconds at differenttemperatures is shown in FIG. 6 (“•” denotes samples dried in vacuumoven for 12 hours at 60° C., and “●” represents samples equilibrated atambient temperature and relative humidity prior to exfoliation).

Note that there is an increase in the surface area as the heatingtemperature increases. Surface areas of 1500 m²/g are achieved byheating the sample at 1030° C. This value is lower than a theoreticalupper surface area of atomically thick graphene sheets, typically 2,600m²/g. Since the adsorption experiment takes place on a bulk TEGO sample,part of the graphene sheets overlap, thus denying access to N₂molecules, resulting in a lower surface area value. An important aspectfor applications involving graphene in polymer matrices is the degree ofdispersion, or the effective surface area, in the dispersed state. TheTEGO materials disperse readily in polar organic solvents such as THF toform a uniform dispersion. Heating temperatures of from about 250-2500°C. may be employed, with a temperature range of from about 500-1500° C.preferred.

The bulk density of a TEGO sample with a surface area of 800 m²/g wasmeasured gravimetrically to be 4.1 kg/m³. Another sample with a measuredsurface area of 1164 m²/g had a bulk density of 1.83 kg/m³.

Example 10

For a comparative study of polymer nanocomposite properties, TEGO,SWCNT, and EG were incorporated into PMMA using solution-basedprocessing methods. Thin-film samples (˜0.1-mm thick) were preparedusing a hot press and fully characterized for thermal, electrical,mechanical, and rheological properties (FIG. 16A). Examination of thefracture surface of EG-PMMA and TEGO-PMMA nanocomposites (FIGS. 16B,16C) reveals an extraordinary difference in the interfacial interactionbetween the polymer matrix and the nanofiller in these two systems.While the multilayer EG fillers protrude cleanly from the fracturesurface indicating a weak interfacial bond, the protruding TEGO platesof the present invention are thickly coated with adsorbed polymerindicating strong polymer-TEGO interaction. The present inventorssuggest that two main differences between EG and TEGO lead to theseinteraction differences: First, distortions caused by the chemicalfunctionalization of the “sp²” graphene sheet and the extremely thinnature of the nanoplates lead to a wrinkled topology at the nanoscale.This nanoscale surface roughness leads to an enhanced mechanicalinterlocking with the polymer chains and consequently, better adhesion.Such an effect is in agreement with the recent suggestion by moleculardynamic studies that show altered polymer mobility due to geometricconstraints at nanoparticle surfaces. Second, while the surfacechemistry of EG is relatively inert, TEGO nanoplates contain pendanthydroxyl groups across their surfaces, which may form hydrogen bondswith the carbonyl groups of the PMMA. Together with TEGO's high surfacearea and nanoscale surface roughness, this surface chemistry is believedto lead to stronger interfacial bonding of TEGO nanoplates with PMMA andthus substantially larger influence on the properties of the hostpolymer.

In polymer nanocomposites, the very high surface-to-volume ratio of thenanoscale fillers provides a key enhancement mechanism that is equallyas important as the inherent properties of the nanofillers themselves.Because the surface area of the nanofiller particles can fundamentallyaffect the properties of the surrounding polymer chains in a regionspanning several radii of gyration surrounding each individualnanoparticle, it is most preferred to have an optimal dispersion of theparticles within the polymer matrix. The high surface area and oxygenfunctional groups in the present invention TEGO nanoplates offer asuperb opportunity to achieve outstanding dispersion and stronginterfacial properties of nanofiller in polymers. While SWCNTs may offersimilar potential without the inherent chemical functionality, inpractice it has proven difficult to extract SWCNTs from their bundles toobtain dispersions to the individual tube level which limits theirenhancement potential.

In FIG. 16A, the thermal and mechanical properties for all three of theaforementioned thin-film samples are provided. Although both glasstransition temperature (T_(g)) and thermal degradation temperature forPMMA increased significantly in the presence of the nanofillers, theTEGO-PMMA nanocomposites significantly outperformed both the EG-PMMA andthe SWCNT-PMMA materials. The glass transition, T_(g), data areparticularly striking: an unprecedented shift of 35° C. occurred for theTEGO-PMMA composite at only 0.05 wt % of the nanofiller. Although theSWCNT-PMMA composite exhibited a broadening of the loss peak, indicatingadditional relaxation modes in the polymer, no significant shift ofT_(g) was observed even at 1 wt % loading. While the SWCNTs were welldistributed in the matrix and well wetted by the polymer, there wasevidence of localized clustering leading to nanotube-rich andnanotube-poor regions in the composite. Consequently the SWCNT-PMMAcomposite retained the rheological signature of bulk PMMA. For theEG-PMMA composite, although no clustering of the EG platelets wasobserved, the platelets were thicker, resulting in a decrease of thesurface area in contact with the polymer and a smaller T_(g) shiftcompared to TEGO-PMMA composites. Functionalization of SWCNTs can leadto better dispersion and a similar T_(g) shift in SWCNT-PMMA composites,but only at 1 wt % loading. Furthermore, functionalizing SWCNTs requiresan additional processing step that is not needed for the TEGO material.In the TEGO nanocomposites, good dispersion of the nanoplate filler andstrong interaction with the matrix polymer resulted in overlappinginteraction zones between the nanoparticles in which the mobility of thepolymer chains was altered, leading to a shift in the bulk T_(g) of thenanocomposite at very low weight fractions.

The room temperature values for tensile Young's modulus (E), ultimatestrength, and the values for storage modulus at elevated temperaturesfollowed a similar trend: the values for TEGO-PMMA exceeded those forSWCNT- and EG-PMMA composites. This increased enhancement in mechanicalproperties for TEGO-PMMA nanocomposite can again be attributed to thesuperior dispersion of the TEGO in the polymer matrix and their intimateinteractions. Even with the partial clustering of the SWCNTs and thelower surface area of the EG platelets, some enhancement of polymerproperties was observed; however, the TEGO nanoplates are believed tofundamentally alter the behavior of the entire polymer matrix even atlow wt % loadings.

While GO itself is electrically non-conducting, an important feature ofTEGO is its substantial electrical conductivity. The longitudinalelectrical conductivity of our TEGO-PMMA nanocomposite greatly surpassesthose of pure PMMA and SWCNT-PMMA nanocomposites (Table 1).

TABLE 1 Electrical conductivity of different nanoscale reinforcements inPMMA at 5 wt % loading. Conductivity measured by AC impedancespectroscopy through the thickness for transverse values and measured bya four-probe steady-state method along the length of the samples forlongitudinal values. DC transverse DC longitudinal conductivity (S/m)conductivity (S/m) PMMA  <1E−10 <1E−10 SWCNT/PMMA 4.7E−03 0.5 EG/PMMA1.1E−03 33.3 TEGO/PMMA 2.9E−02 4.6

That the composites of the present invention approach the conductivityvalue measured for the EG-PMMA system suggests the presence of asignificant conjugated carbon network in the thin TEGO nanoplatesconsistent with the observation that GO underwent partial deoxygenation(reduction) during its rapid high temperature exfoliation into TEGO. Thedata obtained in the comparison also indicate that all threenanocomposite samples were anisotropic, yielding a significantly higherconductivity longitudinally at the same percolation threshold (1-2 wt %level, FIG. 15). For the 5 wt % samples, basic geometric constraintsdictate that the nanoplates cannot be oriented randomly in space. Forflat disks with an aspect ratio of 100, complete random orientation ispossible only at volume fractions less than 5% using an Onsager-typemodel. As the TEGO nanoplates and processed EG have aspect ratios of250-1000, an isotropic arrangement is not possible. This geometricconstraint, combined with the hot-press processing method used toprepare the nanocomposite samples, thus results in partial orientationof the nanoplates parallel to the top and bottom faces of the samples.The EG/PMMA had a higher anisotropy ratio ostensibly due to the morerigid nature of the thicker plates, leading to more longitudinalalignment and higher conductivity. As the conductivity of filledcomposites is controlled by the filler's conductivity and contactresistance between filler particles and the number of filler contacts,it appears that the combination of flexibility and crumpling morphologyof the TEGO plates, together with their exceedingly high aspect ratio,enables percolation at low concentrations. The longitudinal conductivityof the present invention TEGO-PMMA sample was several times that quotedfor 4-6 wt % iodine-doped polyacetylene blended with polyethylene (1S/m).

That the conductivity of 5 wt % TEGO-PMMA composite is quite close tothe conductivity for several commercially important conducting polymersuch as polythiophene and polyaniline opens up potential uses forTEGO-polymer nanocomposites in electronic and photonic applications. Inaddition, since single-layer graphene has been dubbed a zero-gapsemiconductor or small overlap semi-metal as well as the material ofchoice for true nanoscale metallic transistor applications, novelgraphite oxide-derived nanosize conducting materials such as TEGO offervery attractive opportunities indeed.

Example 11

Mechanical Properties of TEGO Filled Polymer Nanocomposites

TEGO/PMMA composites with different weight percentages such as 0.25,0.5, 1, 2, and 5% were prepared using a solution evaporation technique.TEGO/PMMA composite thin films were made using a hot-press moldingmethod. Viscoelastic response of these composites was measured usingDynamic Mechanical Analysis (DMA). Strips of uniform width compositefilm were cut from the film using a razor blade. A tensile force with0.1N pre-load was applied to the test specimen using a ‘film tensionclamp’ in DMA. Then the specimen had applied to it, a dynamicoscillatory force with frequency of 1 Hz. The dynamic properties such asstorage modulus (E′), loss modulus (E″) and tan δ values were measuredwith temperature sweep between 25° C. and 170° C. at the rate of 3°C./min. Storage modulus (E′) vs. temperature response is shown in FIG.17. Storage modulus increment is in the range from 40% for 0.25% weightof TEGO to the maximum of 85% for 1% weight of TEGO than that of PMMA.Further increasing the TEGO concentration decrease the storage modulus.Storage modulus (average taken from 4 or 5 samples for each weightpercent) vs. weight percentage of TEGO is shown in FIG. 18.

It is believed that the reason for decrease in storage modulus forhigher TEGO content may be due to cavities (voids) or clumping ofparticles, which are seen in SEM pictures in FIG. 19. The storagemodulus for expanded graphite in each of (EG)/PMMA and (EG)/PE has beenpreviously shown to increase with filler content up to a few weightpercent. However, the surface of the expanded graphite and TEGO arequite different from each other. The presence of oxide in the surface ofTEGO may create a strong mechanical interaction or interlocking betweenthe polymer and reinforcement particles. In addition, the TEGO plateletsare considerably thinner than the EG plates. Consequently, the limitingvolume fraction for ideal, isolated plate, random dispersion withoutencountering effects of particle clumping will preferably be lower forthe TEGO particles. The samples here at 1 wt % exhibit an increase ofmodulus of nearly 100%, while the published data on EG/PMMA achieved anincrease of only 10%.

FIG. 20 shows that a significant shift is seen in the tan δ peak forTEGO/PMMA composites. The glass transition temperature is normallymeasured using the tan δ peak. It is evident that Tg is nearly 40%higher for TEGO/PMMA composite than pure PMMA, compared to the reportedEG/PMMA composites which showed only a 12%-20% increment in Tg by tan δpeak shift for composites with 1 wt %-3 wt % respectively. Aninteresting feature in Tg is that the tan δ peak shift is nearlyconstant for all volume fractions, but the peak broadens considerablywith the higher volume fractions. Since the Tg is related to themolecular mobility of the polymer, it may considered to be affected bymolecular packing, chain rigidity and linearity. Since the TEGO plateshave a high surface area and thickness on the order of the Rg for apolymer chain, well-dispersed TEGO can have a significant impact on alarge volume fraction of local polymer. In this manner, the interactionof the polymer chains with the surface of the particles can drasticallyalter the chain kinetics in the region surrounding them even at lowerreinforcement content. From FIG. 20, it is evident that chain mobilityis altered at the low concentrations and increasing reinforcementloading appears not to change the major shift in Tg but instead to addadditional relaxation modes, perhaps by interconnectivity of theparticles at higher loadings. The translation of Tg is indicative thatthe TEGO particle interaction with the polymer matrix is nearlyall-inclusive: very little “bulk” polymer remains. A consistent resulton Tg was observed by DSC experiment for these composite samples.

FIG. 21 shows the thermal degradation of the samples. It is clearly seenthat the degradation temperature for the composites are shifted up to15% higher than that of pure polymer. Again, this is viewed as evidencethat the TEGO plates are acting to change the nature of the polymer as awhole in the composite.

AC impedance measurements at room temperature were recorded using aSolartron 1290 impedance analyzer with a 1296 dielectric interface. Thesample was sandwiched between two rectangular copper electrodes withdimension of 21 mm×6 mm held tight to the specimen by two flatpolycarbonate plates. Electrically conductive paste (graphite particlefilled epoxy) was applied between the copper electrode and sample inorder to eliminate the point contacts due to the surface roughness ofthe composite surface. FIG. 22 shows that a significant reduction in thereal Z (resistance) is observed with increasing reinforcement fillercontent. A sharp decease of real Z for 2% and higher TEGO concentrationindicates the onset of electrical percolation. Increase of electricalconductivity has been previously reported for EG/PMMA and graphite/PMMAcomposites over that of pure PMMA. Further the literature suggests thatthe difference in conductivity behavior between EG/PMMA andgraphite/PMMA at higher filler concentration is due to the enhancednumber of conductivity paths in the EG composites. Similar results werereported in HDPE/graphite composites with different filler sizes. Theelectrical conductivity of the present invention composites exhibited apronounced transition with the increase of filler content, from aninsulator to nearly a semiconductor at the percolation limit.

All references cited herein are incorporated by reference.

Numerous modifications and variations on the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

The invention claimed is:
 1. A thermal overload protective device,comprising: a conductive composition comprising: a polymer matrix; andthermally exfoliated graphite oxide configured to: comprise a surfacearea of from about 300 m²/g to 2,600 m²/g; display no signature ofgraphite and no signature of graphite oxide, as determined by X-raydiffraction; and comprise a wrinkle topology at the nanoscale thatmechanically interlocks with the polymer matrix; wherein the conductivecomposition is configured to undergo an event when the conductivecomposition receives a threshold amount of heat or a current; whereinthe event comprises at least one member selected from the groupconsisting of an expansion of the polymer matrix and a decrease inelectrical conductivity.
 2. The thermal overload protective device ofclaim 1, wherein the thermally exfoliated graphite oxide has a bulkdensity of from about 40 kg/m³ to 0.1 kg/m³.
 3. The thermal overloadprotective device of claim 1, wherein the thermally exfoliated graphiteoxide has a C/O ratio of from about 60/40 to 95/5.
 4. The thermaloverload protective device of claim 1, wherein the polymer matrixcomprises a polymer selected from the group consisting of polyethylene,polypropylene and copolymers thereof, polyesters, nylons, polystyrenes,polycarbonates, polycaprolactones, polycaprolactams, fluorinatedethylenes, polyvinyl acetate and its copolymers, polyvinyl chloride,polymethylmethacrylate and acrylate copolymers, high impact polystyrene,styrenic sheet molding compounds, polycaprolactones, polycaprolactams,fluorinated ethylenes, styrene acrylonitriles, polyimides, epoxys,polyurethanes, poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/di(propylene glycol)/polycaprolactone,poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/polytetrahydrofuran, amine terminatedpolybutadienes, carboxyl terminated polybutadienes, polybutadiene,dicarboxy terminated butyl rubber, styrene/butadiene copolymers,polyisoprene, poly(styrene-co-butadiene), polydimethysiloxane, andnatural latex rubber.
 5. The thermal overload protective device of claim1, wherein the conductive composition comprises a thermally exfoliatedgraphite oxide loading level of 0.1 to 90% by weight based on the totalweight of the conductive composition.
 6. The thermal overload protectivedevice of claim 1, wherein the event further comprises a loss ofpercolation of the thermally exfoliated graphite oxide.
 7. A method formaking a thermal overload protective device, comprising the step of:pattering the thermal overload protective device by application of afluid comprising: a polymer; a solvent; and a modified graphite oxidematerial, comprising: thermally exfoliated graphite oxide comprising: asurface area of from about 300 m²/g to 2,600 m²/g; an X-ray diffractionpattern that displays no signature of graphite and no signature ofgraphite oxide; and a wrinkled topology at the nanoscale thatmechanically interlocks with the polymer; wherein the fluid isconfigured to form a conductive composition, the conductive compositionconfigured to undergo an event when the conductive composition receivesheat or a current, the event comprising at least one member selectedfrom the group consisting of an expansion of the polymer matrix and adecrease in electrical conductivity.
 8. The method of claim 7, furthercomprising drying the fluid.
 9. The method of claim 7, wherein thethermally exfoliated graphite oxide has a bulk density of from about 40kg/m³ to 0.1 kg/m³.
 10. The method of claim 7, wherein the thermallyexfoliated graphite oxide has a C/O ratio of from about 60/40 to 95/5.11. The method of claim 7, wherein the polymer comprises materialselected from the group consisting of polyethylene, polypropylene andcopolymers thereof, polyesters, nylons, polystyrenes, polycarbonates,polycaprolactones, polycaprolactams, fluorinated ethylenes, polyvinylacetate and its copolymers, polyvinyl chloride, polymethylmethacrylateand acrylate copolymers, high impact polystyrene, styrenic sheet moldingcompounds, polycaprolactones, polycaprolactams, fluorinated ethylenes,styrene acrylonitriles, polyimides, epoxys, polyurethanes,poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/poly(butylene adipate)],poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/di(propylene glycol)/polycaprolactone,poly[4,4′-methylenebis(phenylisocyanate)-alt-1,4-butanediol/polytetrahydrofuran, amine terminatedpolybutadienes, carboxyl terminated polybutadienes, polybutadiene,dicarboxy terminated butyl rubber, styrene/butadiene copolymers,polyisoprene, poly(styrene-co-butadiene), polydimethysiloxane, andnatural latex rubber.
 12. The method of claim 7, wherein the solventcomprises one or more of water, n-methylpyrolidone (NMP),dimethyformamide (DMF), tetrahydrofuran (THF), an alcohol, a glycol, analiphatic ester, an aromatic ester, a phthalates, a dibutyl phthalate, amethylene chloride, an acetic ester, an aldehyde, a glycol ether, apropionic ester, and a chlorinated solvent.
 13. The method of claim 12,wherein the glycol comprises one or more of ethylene glycol, propyleneglycol, and butylene glycol.
 14. The method of claim 7, wherein theevent further comprises a loss of percolation of the thermallyexfoliated graphite oxide.
 15. The method of claim 7, wherein thesolvent comprises water.